fix(#278): Implement CSRF protection using double-submit cookie pattern

Implemented comprehensive CSRF protection for all state-changing endpoints
(POST, PATCH, DELETE) using the double-submit cookie pattern.

Security Implementation:
- Created CsrfGuard using double-submit cookie validation
- Token set in httpOnly cookie and validated against X-CSRF-Token header
- Applied guard to FederationController (vulnerable endpoints)
- Safe HTTP methods (GET, HEAD, OPTIONS) automatically exempted
- Signature-based endpoints (@SkipCsrf decorator) exempted

Components Added:
- CsrfGuard: Validates cookie and header token match
- CsrfController: GET /api/v1/csrf/token endpoint for token generation
- @SkipCsrf(): Decorator to exempt endpoints with alternative auth
- Comprehensive tests (20 tests, all passing)

Protected Endpoints:
- POST /api/v1/federation/connections/initiate
- POST /api/v1/federation/connections/:id/accept
- POST /api/v1/federation/connections/:id/reject
- POST /api/v1/federation/connections/:id/disconnect
- POST /api/v1/federation/instance/regenerate-keys

Exempted Endpoints:
- POST /api/v1/federation/incoming/connect (signature-verified)
- GET requests (safe methods)

Security Features:
- httpOnly cookies prevent XSS attacks
- SameSite=strict prevents subdomain attacks
- Cryptographically secure random tokens (32 bytes)
- 24-hour token expiry
- Structured logging for security events

Testing:
- 14 guard tests covering all scenarios
- 6 controller tests for token generation
- Quality gates: lint, typecheck, build all passing

Note: Frontend integration required to use tokens. Clients must:
1. GET /api/v1/csrf/token to receive token
2. Include token in X-CSRF-Token header for state-changing requests

Fixes #278

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/app.module.ts

@@ -5,6 +5,7 @@ import { BullModule } from "@nestjs/bullmq";
 import { ThrottlerValkeyStorageService, ThrottlerApiKeyGuard } from "./common/throttler";
 import { AppController } from "./app.controller";
 import { AppService } from "./app.service";
+import { CsrfController } from "./common/controllers/csrf.controller";
 import { PrismaModule } from "./prisma/prisma.module";
 import { DatabaseModule } from "./database/database.module";
 import { AuthModule } from "./auth/auth.module";
@@ -87,7 +88,7 @@ import { FederationModule } from "./federation/federation.module";
     CoordinatorIntegrationModule,
     FederationModule,
   ],
-  controllers: [AppController],
+  controllers: [AppController, CsrfController],
   providers: [
     AppService,
     {

apps/api/src/common/controllers/csrf.controller.spec.ts

@@ -0,0 +1,115 @@
+/**
+ * CSRF Controller Tests
+ *
+ * Tests CSRF token generation endpoint.
+ */
+
+import { describe, it, expect, vi } from "vitest";
+import { CsrfController } from "./csrf.controller";
+import { Response } from "express";
+
+describe("CsrfController", () => {
+  let controller: CsrfController;
+
+  controller = new CsrfController();
+
+  describe("getCsrfToken", () => {
+    it("should generate and return a CSRF token", () => {
+      const mockResponse = {
+        cookie: vi.fn(),
+      } as unknown as Response;
+
+      const result = controller.getCsrfToken(mockResponse);
+
+      expect(result).toHaveProperty("token");
+      expect(typeof result.token).toBe("string");
+      expect(result.token.length).toBe(64); // 32 bytes as hex = 64 characters
+    });
+
+    it("should set CSRF token in httpOnly cookie", () => {
+      const mockResponse = {
+        cookie: vi.fn(),
+      } as unknown as Response;
+
+      const result = controller.getCsrfToken(mockResponse);
+
+      expect(mockResponse.cookie).toHaveBeenCalledWith(
+        "csrf-token",
+        result.token,
+        expect.objectContaining({
+          httpOnly: true,
+          sameSite: "strict",
+        })
+      );
+    });
+
+    it("should set secure flag in production", () => {
+      const originalEnv = process.env.NODE_ENV;
+      process.env.NODE_ENV = "production";
+
+      const mockResponse = {
+        cookie: vi.fn(),
+      } as unknown as Response;
+
+      controller.getCsrfToken(mockResponse);
+
+      expect(mockResponse.cookie).toHaveBeenCalledWith(
+        "csrf-token",
+        expect.any(String),
+        expect.objectContaining({
+          secure: true,
+        })
+      );
+
+      process.env.NODE_ENV = originalEnv;
+    });
+
+    it("should not set secure flag in development", () => {
+      const originalEnv = process.env.NODE_ENV;
+      process.env.NODE_ENV = "development";
+
+      const mockResponse = {
+        cookie: vi.fn(),
+      } as unknown as Response;
+
+      controller.getCsrfToken(mockResponse);
+
+      expect(mockResponse.cookie).toHaveBeenCalledWith(
+        "csrf-token",
+        expect.any(String),
+        expect.objectContaining({
+          secure: false,
+        })
+      );
+
+      process.env.NODE_ENV = originalEnv;
+    });
+
+    it("should generate unique tokens on each call", () => {
+      const mockResponse = {
+        cookie: vi.fn(),
+      } as unknown as Response;
+
+      const result1 = controller.getCsrfToken(mockResponse);
+      const result2 = controller.getCsrfToken(mockResponse);
+
+      expect(result1.token).not.toBe(result2.token);
+    });
+
+    it("should set cookie with 24 hour expiry", () => {
+      const mockResponse = {
+        cookie: vi.fn(),
+      } as unknown as Response;
+
+      controller.getCsrfToken(mockResponse);
+
+      expect(mockResponse.cookie).toHaveBeenCalledWith(
+        "csrf-token",
+        expect.any(String),
+        expect.objectContaining({
+          maxAge: 24 * 60 * 60 * 1000, // 24 hours
+        })
+      );
+    });
+  });
+});

apps/api/src/common/controllers/csrf.controller.ts

@@ -0,0 +1,35 @@
+/**
+ * CSRF Controller
+ *
+ * Provides CSRF token generation endpoint for client applications.
+ */
+
+import { Controller, Get, Res } from "@nestjs/common";
+import { Response } from "express";
+import * as crypto from "crypto";
+import { SkipCsrf } from "../decorators/skip-csrf.decorator";
+
+@Controller("api/v1/csrf")
+export class CsrfController {
+  /**
+   * Generate and set CSRF token
+   * Returns token to client and sets it in httpOnly cookie
+   */
+  @Get("token")
+  @SkipCsrf() // This endpoint itself doesn't need CSRF protection
+  getCsrfToken(@Res({ passthrough: true }) response: Response): { token: string } {
+    // Generate cryptographically secure random token
+    const token = crypto.randomBytes(32).toString("hex");
+
+    // Set token in httpOnly cookie
+    response.cookie("csrf-token", token, {
+      httpOnly: true,
+      secure: process.env.NODE_ENV === "production",
+      sameSite: "strict",
+      maxAge: 24 * 60 * 60 * 1000, // 24 hours
+    });
+
+    // Return token to client (so it can include in X-CSRF-Token header)
+    return { token };
+  }
+}

apps/api/src/common/decorators/skip-csrf.decorator.ts

@@ -0,0 +1,20 @@
+/**
+ * Skip CSRF Decorator
+ *
+ * Marks an endpoint to skip CSRF protection.
+ * Use for endpoints that have alternative authentication (e.g., signature verification).
+ *
+ * @example
+ * ```typescript
+ * @Post('incoming/connect')
+ * @SkipCsrf()
+ * async handleIncomingConnection() {
+ *   // Signature-based authentication, no CSRF needed
+ * }
+ * ```
+ */
+
+import { SetMetadata } from "@nestjs/common";
+import { SKIP_CSRF_KEY } from "../guards/csrf.guard";
+
+export const SkipCsrf = () => SetMetadata(SKIP_CSRF_KEY, true);

apps/api/src/common/guards/csrf.guard.spec.ts

@@ -0,0 +1,140 @@
+/**
+ * CSRF Guard Tests
+ *
+ * Tests CSRF protection using double-submit cookie pattern.
+ */
+
+import { describe, it, expect, beforeEach, vi } from "vitest";
+import { ExecutionContext, ForbiddenException } from "@nestjs/common";
+import { Reflector } from "@nestjs/core";
+import { CsrfGuard } from "./csrf.guard";
+
+describe("CsrfGuard", () => {
+  let guard: CsrfGuard;
+  let reflector: Reflector;
+
+  beforeEach(() => {
+    reflector = new Reflector();
+    guard = new CsrfGuard(reflector);
+  });
+
+  const createContext = (
+    method: string,
+    cookies: Record<string, string> = {},
+    headers: Record<string, string> = {},
+    skipCsrf = false
+  ): ExecutionContext => {
+    const request = {
+      method,
+      cookies,
+      headers,
+      path: "/api/test",
+    };
+
+    return {
+      switchToHttp: () => ({
+        getRequest: () => request,
+      }),
+      getHandler: () => ({}),
+      getClass: () => ({}),
+      getAllAndOverride: vi.fn().mockReturnValue(skipCsrf),
+    } as unknown as ExecutionContext;
+  };
+
+  describe("Safe HTTP methods", () => {
+    it("should allow GET requests without CSRF token", () => {
+      const context = createContext("GET");
+      expect(guard.canActivate(context)).toBe(true);
+    });
+
+    it("should allow HEAD requests without CSRF token", () => {
+      const context = createContext("HEAD");
+      expect(guard.canActivate(context)).toBe(true);
+    });
+
+    it("should allow OPTIONS requests without CSRF token", () => {
+      const context = createContext("OPTIONS");
+      expect(guard.canActivate(context)).toBe(true);
+    });
+  });
+
+  describe("Endpoints marked to skip CSRF", () => {
+    it("should allow POST requests when @SkipCsrf() is applied", () => {
+      vi.spyOn(reflector, "getAllAndOverride").mockReturnValue(true);
+      const context = createContext("POST");
+      expect(guard.canActivate(context)).toBe(true);
+    });
+  });
+
+  describe("State-changing methods requiring CSRF", () => {
+    it("should reject POST without CSRF token", () => {
+      const context = createContext("POST");
+      expect(() => guard.canActivate(context)).toThrow(ForbiddenException);
+      expect(() => guard.canActivate(context)).toThrow("CSRF token missing");
+    });
+
+    it("should reject PUT without CSRF token", () => {
+      const context = createContext("PUT");
+      expect(() => guard.canActivate(context)).toThrow(ForbiddenException);
+    });
+
+    it("should reject PATCH without CSRF token", () => {
+      const context = createContext("PATCH");
+      expect(() => guard.canActivate(context)).toThrow(ForbiddenException);
+    });
+
+    it("should reject DELETE without CSRF token", () => {
+      const context = createContext("DELETE");
+      expect(() => guard.canActivate(context)).toThrow(ForbiddenException);
+    });
+
+    it("should reject when only cookie token is present", () => {
+      const context = createContext("POST", { "csrf-token": "abc123" });
+      expect(() => guard.canActivate(context)).toThrow(ForbiddenException);
+      expect(() => guard.canActivate(context)).toThrow("CSRF token missing");
+    });
+
+    it("should reject when only header token is present", () => {
+      const context = createContext("POST", {}, { "x-csrf-token": "abc123" });
+      expect(() => guard.canActivate(context)).toThrow(ForbiddenException);
+      expect(() => guard.canActivate(context)).toThrow("CSRF token missing");
+    });
+
+    it("should reject when tokens do not match", () => {
+      const context = createContext(
+        "POST",
+        { "csrf-token": "abc123" },
+        { "x-csrf-token": "xyz789" }
+      );
+      expect(() => guard.canActivate(context)).toThrow(ForbiddenException);
+      expect(() => guard.canActivate(context)).toThrow("CSRF token mismatch");
+    });
+
+    it("should allow when tokens match", () => {
+      const context = createContext(
+        "POST",
+        { "csrf-token": "abc123" },
+        { "x-csrf-token": "abc123" }
+      );
+      expect(guard.canActivate(context)).toBe(true);
+    });
+
+    it("should allow PATCH when tokens match", () => {
+      const context = createContext(
+        "PATCH",
+        { "csrf-token": "token123" },
+        { "x-csrf-token": "token123" }
+      );
+      expect(guard.canActivate(context)).toBe(true);
+    });
+
+    it("should allow DELETE when tokens match", () => {
+      const context = createContext(
+        "DELETE",
+        { "csrf-token": "delete-token" },
+        { "x-csrf-token": "delete-token" }
+      );
+      expect(guard.canActivate(context)).toBe(true);
+    });
+  });
+});

apps/api/src/common/guards/csrf.guard.ts

@@ -0,0 +1,83 @@
+/**
+ * CSRF Guard
+ *
+ * Implements CSRF protection using double-submit cookie pattern.
+ * Validates that CSRF token in cookie matches token in header.
+ *
+ * Usage:
+ * - Apply to controllers handling state-changing operations
+ * - Use @SkipCsrf() decorator to exempt specific endpoints
+ * - Safe methods (GET, HEAD, OPTIONS) are automatically exempted
+ */
+
+import {
+  Injectable,
+  CanActivate,
+  ExecutionContext,
+  ForbiddenException,
+  Logger,
+} from "@nestjs/common";
+import { Reflector } from "@nestjs/core";
+import { Request } from "express";
+
+export const SKIP_CSRF_KEY = "skipCsrf";
+
+@Injectable()
+export class CsrfGuard implements CanActivate {
+  private readonly logger = new Logger(CsrfGuard.name);
+
+  constructor(private reflector: Reflector) {}
+
+  canActivate(context: ExecutionContext): boolean {
+    // Check if endpoint is marked to skip CSRF
+    const skipCsrf = this.reflector.getAllAndOverride<boolean>(SKIP_CSRF_KEY, [
+      context.getHandler(),
+      context.getClass(),
+    ]);
+
+    if (skipCsrf) {
+      return true;
+    }
+
+    const request = context.switchToHttp().getRequest<Request>();
+
+    // Exempt safe HTTP methods (GET, HEAD, OPTIONS)
+    if (["GET", "HEAD", "OPTIONS"].includes(request.method)) {
+      return true;
+    }
+
+    // Get CSRF token from cookie and header
+    const cookies = request.cookies as Record<string, string> | undefined;
+    const cookieToken = cookies?.["csrf-token"];
+    const headerToken = request.headers["x-csrf-token"] as string | undefined;
+
+    // Validate tokens exist and match
+    if (!cookieToken || !headerToken) {
+      this.logger.warn({
+        event: "CSRF_TOKEN_MISSING",
+        method: request.method,
+        path: request.path,
+        hasCookie: !!cookieToken,
+        hasHeader: !!headerToken,
+        securityEvent: true,
+        timestamp: new Date().toISOString(),
+      });
+
+      throw new ForbiddenException("CSRF token missing");
+    }
+
+    if (cookieToken !== headerToken) {
+      this.logger.warn({
+        event: "CSRF_TOKEN_MISMATCH",
+        method: request.method,
+        path: request.path,
+        securityEvent: true,
+        timestamp: new Date().toISOString(),
+      });
+
+      throw new ForbiddenException("CSRF token mismatch");
+    }
+
+    return true;
+  }
+}

apps/api/src/federation/federation.controller.ts

@@ -12,6 +12,8 @@ import { FederationAuditService } from "./audit.service";
 import { ConnectionService } from "./connection.service";
 import { AuthGuard } from "../auth/guards/auth.guard";
 import { AdminGuard } from "../auth/guards/admin.guard";
+import { CsrfGuard } from "../common/guards/csrf.guard";
+import { SkipCsrf } from "../common/decorators/skip-csrf.decorator";
 import type { PublicInstanceIdentity } from "./types/instance.types";
 import type { ConnectionDetails } from "./types/connection.types";
 import type { AuthenticatedRequest } from "../common/types/user.types";
@@ -25,6 +27,7 @@ import {
 import { FederationConnectionStatus } from "@prisma/client";
 
 @Controller("api/v1/federation")
+@UseGuards(CsrfGuard)
 export class FederationController {
   private readonly logger = new Logger(FederationController.name);
 
@@ -38,6 +41,7 @@ export class FederationController {
    * Get this instance's public identity
    * No authentication required - this is public information for federation
    * Rate limit: "long" tier (200 req/hour) - public endpoint
+   * CSRF exempt: GET method (safe)
    */
   @Get("instance")
   @Throttle({ long: { limit: 200, ttl: 3600000 } })
@@ -207,8 +211,10 @@ export class FederationController {
    * Handle incoming connection request from remote instance
    * Public endpoint - no authentication required (signature-based verification)
    * Rate limit: "short" tier (3 req/sec) - CRITICAL DoS protection (Issue #272)
+   * CSRF exempt: Uses signature-based authentication instead
    */
   @Post("incoming/connect")
+  @SkipCsrf()
   @Throttle({ short: { limit: 3, ttl: 1000 } })
   async handleIncomingConnection(
     @Body() dto: IncomingConnectionRequestDto